Tube Amps / Music Electronics
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|previous: regis Hi Mike,||View Thread|
|1/5/2004 9:22 PM|
|MBSetzer||Re: Hum in Silvertone 1482|
remember that the tone controls arranged like this are just treble-cut knobs so you mainly need to just keep them both maximized at all times. My tone pots were only about 750K so I also added a 330K in series with each one to reduce the loss of highs when they are maximized.
Then in case you are not leaving the volume controls both maximized as well, you might benefit from a bright cap of about 100 to 150pf silver-mica across each volume pot, so that the highest highs (those which can make it through the cap) do not get attenuated as you reduce the volume knobs.
Weak output can be categorized as either truly low power, or as lack of gain.
Bad filter caps, especially the 20uf section of the can marked C17C, can result in low power even if the hum is not loud enough to be annoying. These narrow-can electrolytics have not been made in years, but amazingly mine was not worn out in a way that yielded excessive electrical leakage, no it just had lost most of its capacitance over the decades. So I left the cap there and added a 47uf in parallel to C17C, and a 22uf in parallel to C17B, and another 22uf in parallel with C17A. Plus 4.7uf in parallel with C1 so the preamp had a little more filtering. Plus once I got really going, I noticed the way that the plate supply of the 6AU6 is connected directly to the screens of the 6V6's, and there were not any screen stopper resistors either. So I put a 390ohm resistor on the screen pin of each 6V6, then after they join C17B, added a 1.5K resistor between there and R30, plus a new 4.7uf electrolytic to ground where the new 1.5K meets R30. This gives the 6AU6 tremolo oscillator its own little filtered power supply node. Also paralleled a 2.7K with R35 after adding the extra power supply filtration, plus replaced R15 with a 3K and that brings up the preamp & PI voltages to respectable levels.
Another place where a weak old cap results in low power is C9, the 10uf/25V cathode bypass cap for the 6V6's. Plus it seems like a better idea to use a cap rated for more than 25V on the cathode of a 6V6, I replaced the original with 15uf & 50V rating.
One more thing about the power, you do not want a rectifier with a weak section in it. This circuit is really asking a bit much from such a small rectifier as a 6X4 to begin with, it does not need to be brand new but can not be weak. I had about a dozen used ones and a couple NOS which I tested plus listened to the difference. There were only 2 that were unsuitable and it was obvious on the tube tester that one plate was weaker than the other. Other than that the used 6X4's sounded as good as the NOS.
When it comes to gain, the 330K plate resistors on the first 12AX7 do give more gain than if they were 100K, but the 2.2K cathode resistors on these two triodes do not have bypass caps, so that limits the overall gain of both parallel input stages. Here is where I made the first major modifications to get a little more conventional Fender sound, I changed one channel to have a 100K plate and a 1.5K cathode just like a Fender, but I put 4.7uf bypass cap on the 1.5K instead of the fender 25uf value. Then on the other channel I paralleled another 330K with the one that was there to result in about 160 to 170K at the plate of that channel, and left the original 2.2K cathode resistor there, also bypassing that one with a 4.7uf film cap.
So each channel now has a little bit different response, and that gives greater variation when jumping them and blending them, plus when you only play through one at a time at least you have two different channels to choose from.
Then the second and final gain stage where R16, the 2.2K cathode resistor is also not bypassed, I put a 4.7uf there as well. This is the first one that I made switchable as a booster function for either or both channels, regardless of where you plug into.
Also, where the schematic shows R23 & R25 as 330K grid stoppers, mine did not have any resistors here originally. Experience has shown that values as high as 330K can really tame the brightness on some amps, I put in 100K for each grid which is a value that works well for me with the level of preamp distortion I am generating upstream of there.
Anyway, this circuit is already close enough to a 5E3 to where the Fender could be copied exactly and also have the trem feature, but it would still not have the same transformers as a Fender nor would it be built of as good materials and will never sound exactly the same. IMHO this is an excellent opportunity not to be restricted by the legendariness of the 5E3 circuit, especially since the chassis & cabinet are so much different. An amp which looks like or has the same chassis & cab dimensions as a 5E3 would not be generally as desirable without a close copy of the 5E3 circuit inside. The same circuit changes that would make a real 5E3 more suitable for yours or my playing would still not be a very good idea to do to a vintage 5E3 even in poor condition, they should probably just be restored to the original circuit & values. With the Silvertone though, just about anything goes so once you get it to where its equally suitable for you as much as a 5E3, you can keep on tweaking until its truly better. At least regarding the tone, the chassis is never going to be robust and the cabinet is so unroadworthy that you really have to be careful not to let anyone sit on it, not even once
|Ben N Mike:|
Ben N Another thought/Q: